416 



ME. H. M. BEENARD ON THE 



[Aug. 1894, 



and solely from an examination of the external and internal 

 structure of Apus, that such bending round must have taken place 

 in the ancestral crustacean (' The Apodidae,' op. supra cit.). But, 

 having only Apus as a guide, I had to leave the question undecided 

 as to how many segments actually turned over, i. e. into or towards 



Fig. 4. — Olenellus asaphoides, after Ford. 



[The order has been accidentally reversed.] 



a=Embryonic form : head composed of five segments, which diminish in size 



from before backward. 

 b = A further stage of the same. 

 c= Pleurae of the fifth segment beginning to take part in the formation of the 



head-sliield. 



the horizontal plane. Finding the mandibles (belonging to the 

 third segment) arranged dorso-ventrally, I have since been inclined 

 to think that the third segment remained more or less completely 

 in the transverse plane. I should therefore have assigned the chief 

 part in the formation of the bend to the first and second segments. 



A study of Olenellus, in which the segmentation of the head is 

 especially distinct, shows us that such a bending round did actually 

 take place, but that it was primarily confined to one, i. e. to the 

 first, segment. By the bending 

 round of the first segment, so 

 that the labrum and mouth 

 point backward, thus apparent 

 in the trilobites, we can, as I 

 have shown, obtain an expla- 

 nation of the pre-oral position 

 of the antennae in the Crustacea, 

 and, further, of the bend in the 

 alimentary canal also charac- 

 teristic of the group, and espe- 

 cially marked in Apus and 

 Limulus. In this latter animal, 

 indeed, the backward bend of 

 the oesophagus has been second- 

 arily exaggerated. The same may also have taken place in 

 Apus. If so, it must be attributed to the gradual backward growth 

 of the mouth, so as to allow a greater number of limbs to function 

 as mouth-parts. In Limulus the basal plates of five pairs of limbs 



Fig. 5.— Young specimen of 

 Olenellus asaphoides. 



[The pleurae of the fifth head-segment 

 are seen to resemble those of the 

 trunk-segments.] 



